1. Field of the Invention
The present invention relates generally to an apparatus for controlling the steering angle of the rear wheels of an automotive vehicle, on the basis of parameters which include the steering angle of the front wheels and the running speed of the vehicle. More particularly, the invention is concerned with an arrangement for determining the desired value of the steering angle of the rear wheels.
2. Discussion of the Prior Art
A vehicle rear-wheel steering-angle controlling apparatus of the type indicated above usually includes (a) front-wheel steering angle obtaining means for obtaining or detecting the steering angle of the front wheels of a vehicle, (b) vehicle-speed obtaining means for obtaining or detecting the running speed of the vehicle, (c) angle determining means for determining a desired or target value of the steering angle of the rear wheels, on the basis of operating parameters of the vehicle which include the obtained steering angle of the front wheels and the obtained vehicle running speed, so that the actual steering angle of the rear wheels coincides with the determined target value, (d) an actuator for changing the steering angle of the rear wheels, and (e) actuator driving means for driving the actuator to establish the desired value of the steering angle of the rear wheels.
An example of the rear-wheel steering-angle controlling apparatus of the type described above is disclosed in JP-B-1-14068. The apparatus as disclosed in this publications adapted to change the desired steering angle of the rear wheels as the vehicle running speed is changed as a result of an operation of an accelerator or brake pedal or other operator-controlled member for accelerating or decelerating the vehicle.
However, the known apparatus described above suffers from a problem as described below.
The vehicle runs with the steering angle of the front wheels being controlled by a vehicle operator by means of a steering wheel. The steering wheel is either operated to drive the vehicle along a curve, or held non-operated with the front-wheel steering angle kept substantially constant. Namely, the steering wheel has a transient state in which the steering rate at which the steering wheel is operated is higher than a predetermined threshold (which is close to zero), and a steady state in which the steering rate is equal to or lower than the threshold, for example, the steering rate is substantially zero. In the known apparatus, the steering angle of the rear wheels changes with a change in the vehicle running speed, even in the steady state of the steering wheel. Accordingly, the attitude or orientation of the vehicle on a plane parallel to the road surface, with respect to the running direction of the vehicle may be changed even when the steering wheel is held in substantially the same angular operating position.
Usually, while the vehicle is running in the steady state of the steering wheel, the operator has a better drive feeling when the steering angle of the rear wheels is held constant, unless the rate of change in the vehicle speed is relatively high. It is generally true that the vehicle operator wants to maintain the same attitude or orientation of the vehicle when the operator is holding the steering wheel at the same angular position. Thus, the known apparatus gives the vehicle operator a considerable feeling of discomfort in the steady state of the steering wheel that the manipulation of the steering wheel by the operator does not accurately reflect the actual change in the steering angle of the rear wheels and in the orientation of the vehicle with respect to the desired running direction of the vehicle.